Moisture curing urethane prepolymers have found their applications in adhesives, sealants, coatings and binders areas. Specifically, they can either be liquid prepolymers or hot melts. Curable hot melt adhesives and sealants are solid materials at room temperature and applied in molten form at high temperature. On cooling, the material solidifies and subsequently cures by a chemical crosslinking reaction to a form with good strength and toughness. Advantages of hot melt curable adhesives and sealants over non-curing hot melt systems include improved temperature and chemical resistance. An advantage of hot melt curable adhesives over traditional liquid curing systems is their ability to provide “green strength” upon cooling prior to cure.
The majority of reactive hot melts are moisture-curing urethane systems. They consist of isocyanate terminated urethane prepolymers that react with surface or ambient moisture in order to chain-extend and branch, forming a new polyurethane/urea polymer. Polyurethane prepolymers are conventionally obtained by reacting polyols with isocyanates. Cure is obtained through the diffusion of moisture from the atmosphere or the substrates into the prepolymer, and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is a crosslinked material polymerized primarily through urea groups and urethane groups. Hot melt adhesives and sealants, particularly those known as “reactive” or “moisture-curing” hot melt have characteristics which make them well suited for many applications which require high strength bonds that will withstand harsh environmental conditions.
Urethane prepolymer is used in many industrial applications where semistructural or even structural performance is required. Traditionally, hot melt offers better processing characteristics than one-part moisture curing liquid urethane system or two-part urethane. However, at lower crosslinking density, the heat resistance of cured hot melt is generally inferior to the other two types of urethane systems. It limits the application of hot melt urethane adhesives and sealants in key areas where heat resistance is important. There has been a constant drive to improve the high temperature mechanical property of hot melt to expand its application window.
There are several ways to improve the heat resistance of hot melt. These methods include the use of crystalline polyester polyols and thermoplastic materials, and modification of the cross-linking density of the hot melt.
Current technology formulates with crystalline polyester polyols that has high mechanical strength at room temperature. The resulting reactive hot melt has good mechanical performance after fully cured, but the heat stability is limited by the mechanical strength of the polyester. Upon heating, when the backbone of polyester reaches its transition temperature, the mechanical strength of the cured urethane system suffers.
When thermoplastic material is added, it can contribute to the heat resistance if its Tg is higher than the end use temperature and the molecular weight is higher than its entangled molecular weight. However, to achieve heat resistance of 80-100° C., a high molecular weight thermoplastic polymer with a glass transition temperature higher than 90° C. is needed. High molecular weight and high Tg may limit the polymer's compatibility with the rest of the system. It will cause processing difficulties in the manufacture and in the subsequent application of the adhesive.
The cross-linking density also contributes to the heat resistance. For example, moisture curing one-part liquid urethanes have high cross-linking density and good heat resistance. However, increasing the cross-linking density of hot melt systems requires higher isocyanate group concentration, which results in safety and health concerns and may cause excess bubbling that affect final bond properties. The stability of the adhesive in typical hot melt application equipment also suffers as the high NCO concentration of the prepolymer leads to fast viscosity increase at the application temperature. Therefore it is undesirable to improve the heat resistance of hot melt systems by increasing isocyanate group concentration.
There is a need in the art for high heat resistance moisture curing urethane prepolymer compositions that offer a good balance of open time, green strength, and favorable application properties. The current invention addresses this need.